1. Field of the Invention
The present invention relates to a laser beam printer which deflects a laser beam, modulated by an image signal, using deflecting means such as a rotating polygon mirror in order to scan a photosensitive member.
2. Description of the Related Art
FIG. 13 illustrates the construction of a conventional laser beam printer, and will be used to describe image formation by a laser beam printer.
Reference numeral 101 denotes an image signal (VDO signal) which is input to a laser beam unit 102. Reference numeral 103 denotes a laser beam subjected to on/off modulation by the aforementioned laser beam unit 102. Reference numeral 104 denotes a scanner motor which rotates a rotating polygon mirror 105. Reference numeral 106 denotes an image forming lens which focuses the laser beam 103, deflected by the polygon mirror 105, on a photosensitive drum 108.
The laser beam 103, modulated by the image signal 101 and deflected by the polygon mirror 105, horizontally scans the photosensitive drum 108 along the main scanning dimension. Reference numeral 109 denotes an optical/electrical conversion device which outputs a photoelectric conversion signal 110 upon irradiation by the laser beam 103. (This signal 110 will hereinafter be referred to as the "BD signal"). The BD signal 110 is transmitted to an image controller (not shown) which synchronizes with the BD signal 110 in order to synchronize the input of the image signal 101. Reference numeral 112 denotes a transfer paper used by a transfer device (not shown) for transferring a toner image thereon, the toner image being a latent image formed on the photosensitive drum 108 and being made visible by a developing device (not shown).
A description will now be given of controlling signals utilized for image formation, with reference to FIG. 14.
In transferring a toner image formed on the photosensitive drum 108 onto the transfer paper 112, a laser beam exposable area (or image forming area) 122 is provided on the photosensitive drum 108 so that the entire formed toner image is transferred onto the transfer paper 112. The image controller (not shown) which outputs the image signal 101 is often not a component of the apparatus that comprises the engine control section, which controls the printer engine, but is a component of a different apparatus, or of an external computer. When the image controller turns on the image signal 101 in a non-image region of the photosensitive drum 108 and the laser beam 103 is turned on, the resulting toner image is not transferred to the transfer paper 112. This causes staining in the printer and on the reverse side of the transfer paper. To solve this problem, the laser beam 103 is not turned on when an image signal 101 is turned on outside the image forming area 122. The size of the image forming area 122 is determined by the size of the transfer paper 112.
A description will now be given of the signals required for forming an image corresponding to a main scanning line 123 onto the transfer paper 112. As described earlier, the BD signal 110 is a main scanning dimension synchronization signal, therefore other signals are generated so as to be in synchronism with the BD signal 110.
The mask signal 125 is a signal which is turned on and off in accordance with the image forming area 122, and which intercepts the image signal 101 when scanning is performed outside the image forming area in order to prohibit exposure of the photosensitive drum 108 outside the image forming area 122.
The BD input permission signal 127 is a signal which permits input of the BD signal 110. This signal does not allow the BD signal 110 to be accepted until a predetermined period of time from the previous BD signal 110. Accordingly, the BD input permission signal 127 masks the BD signal 110 for a predetermined time period to prevent positional displacement along the main scanning dimension.
The unblanking signal 128 is a timing signal for turning on the laser when the laser beam is employed to scan the detection device 109, which generates the BD signal 110. Like the BD input permission signal 127, the unblanking signal 128 is a signal which is generated for a predetermined period of time from the input of the previous BD signal 110.
A description will now be given of the circuits generating a control signal along the main scanning dimension, with reference to FIG. 15. Reference numeral 16 denotes a CPU for controlling the scanning sequence of the laser beam printer, not only along the main scanning dimension but also along the sub-scanning dimension, and for performing other operations. In FIG. 15, circuits other than those required for main scanning control are not shown. Reference numeral 1 denotes an address bus which selects registers 2 via an address decoder 15. Reference numeral 9 denotes a data bus used in writing data from the CPU 16 to each register 2 or reading data from each register 2 into the CPU 16. Each register 2 is provided for storing a count value for generating or terminating the mask signal 125, the BD input permission signal 127, and the unblanking signal 128.
The count values, stored in the registers 2, are compared with the content of the main scanning counter 13, which starts counting upon generation of the BD signal 110 and thereby generates the previously mentioned signals. Comparators 3 are provided to compare the count values, stored in the various registers 2, with the value of the main scanning counter 13. Reference numeral 18 denotes J/K flip-flop sections which synthesize various start timing signals and end timing signals to generate the mask signal 125 and the unblanking signal 128, which are sent to an image control signal generating section 17. Based on these signals, a laser emitting permission signal 6 and a forced laser lighting signal 7 are sent from the image control signal generating section 17 to the laser unit.
On the other hand, the BD input permission signal 127 is sent to a BD signal masking circuit 19, which accepts the BD signal 110 only when the BD input permission signal 127 is active.
The scanner motor 104, causing laser beam scanning, is controlled so as to synchronize an FG signal generated in proportion to the rotating speed with a reference clock by speed feedback or phase feedback (PLL control). Although, in general, FG signals are used to detect the rotation, BD signals may also be used to increase rotation precision quite easily.
However, when the rotation of the scanner motor 104 is controlled by, for example, a BD signal 110, the laser must be turned on before the scanner motor 104 starts rotating at a fixed speed in order to generate the BD signal 110, which gives rise to the following problems.
Since it is necessary to turn on the laser from the time before the scanner motor starts rotating at a fixed speed, a beam of slow scanning speed irradiates the photosensitive drum 108. Thus, the photosensitive drum 108 receives a greater amount of energy than is received when forming an ordinary image, which deteriorates part of the drum.
In controlling the scanner motor using a BD signal, it is necessary to intermittently turn on the laser during the start-up time of the scanner motor. Although the start-up time of the scanner motor is ordinarily a few seconds, these seconds add up to a length of time large enough to decrease the lifetime of the laser chip in the case where intermittent printing is to be performed.
In addition, the shorter the active period of the BD input permission signal 127, the less likely is erroneous detection of the BD signal 110 due to noise. However, if this period becomes too short, and if imprecise surface division of the rotating polygon mirror results in a longer scanning time (main scanning period) of the surface than expected, the BD signal 110 generated outside the BD input permission range may not be detected, even when the scanner motor has shifted only by a small amount.
FIG. 16 illustrates a path taken by the BD signal when a scanner motor has shifted. With the waveform of the BD signal denoted by reference numeral 61, the BD signal path is as denoted by reference numeral 63 along the vertical time axis (along the direction denoted by reference numeral 62). The broken lines, denoted by reference numeral 64, indicate the time period the BD input permission signal is permitting the input of the BD signal 110. The shorter this time period, the less the printer is affected by noise introduced into BD signals, etc. However, as indicated by the path 63, when the BD signal 110 strikes an area outside the input permission range, the BD signal 110 cannot be detected, even when the scanner motor has shifted by only a small amount. In recent years, this problem has been particularly apparent with the realization of high-speed image formation because of greater noise effects resulting from the use of a high-frequency clock or the like.